Hi Dan,
Some great animations!
Firstly, I should set the scene as to why I posed the original question.
If you go back to the original question asked by James Setzler, K1SD on
14/11/13, James was talking particularly about the 80mx and 40mx
performance of his HF2V.
The excellent work that Jim K9YC did with his analysis and presentation -
everything I agree with, both from a theoretical and a practical (50 years
DXing) point of view - but remember that this analysis is for 40mx, 20mx and
10mx.
What needs to be considered is that on 80mx (also 160mx) and to a lesser
extent 40mx, the angle of arrival and takeoff for DX signals is actually
very much higher than one would expect.
The ARRL antenna handbook for example, quotes figures like: for 40mx
99% of the time signal below 35 degrees, 50% of time above 25 degrees and
99% of the time above 10 degrees.
These figures have been backed up by countless research papers over the
years
For 80mx and 160mx the signals are at even greater elevation angles. (and
I should add, varies with time as the ionosphere rises/tilts/ Dlayer
appears, sporadic clouds in summer etc. etc.)
******************
Although there are many other factors to consider with ionospheric
propagation, if we take for example Jim's analysis
http://k9yc.com/VerticalHeight.pdf of a 40mx vertical ground plane antenna
at ground level, 33 feet and 40 feet, we find that:
City ground from 33 -> 50 degrees the ground level antenna outperforms the
33 feet antenna
Rocky ground from 25 -> 60 degrees the ground level antenna outperforms
the 33 feet antenna
Average ground from 23 -> 57 degrees the ground level antenna outperforms
the 33 feet antenna
Pastoral ground from 19 -> 57 degrees the ground level antenna outperforms
the 33 feet antenna
Very good ground from 17 -> 60 degrees the ground level antenna
outperforms the 33 feet antenna
As I have said, there are many other factors to consider, but with some 50%
of signals often arriving and departing at above 25 degrees, it is really
only city situations where you can clearly state that the elevated ground
plane will almost always outperform the ground mounted antenna.
>From years of experience using vertical antennas, this high angle effect on
lower frequency signals is definitely something that needs to be considered
and especially on 80mx (and 160mx).
Jim's excellent analysis shows quite clearly what the trend is as you move
from poor to good local ground conditions.
My question related to just how effective measuring vertically polarised
signals at approximately zero elevation angle is with regard to DXing on
80mx and 40mx.
Given the choice, most would opt for an elevated vertical. However on 80mx
and to a lesser extent on 40mx a ground mounted antenna on good ground can
be an effective DX antenna.
Cheers
Peter VK3QI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Dan
Maguire
Sent: Friday, 15 November 2013 9:15 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] K9YC vertical dipole test....
VK3QI wrote:
>>> Are you really measuring what DXers are really after when comparing
vertical antennas at different heights above ground and ground mounted.?
Putting aside for the moment the distinction between measuring and modeling,
I wanted to investigate what VK3QI said about "DX angles" (in this case 10°)
vs "zero" angle. First thing I did was create an adjustable vertical dipole
model that also has ground conditions that can be set via a variable.
Here's the AutoEZ "Variables" sheet showing (to start) the base (variable B)
at 20 ft, the length (variable D) at 35 ft, and the ground type (variable G)
as "Average".
http://ac6la.com/adhoc/VertDip1.png
With those starting conditions I used the Resonate button to reset the
length (variable D). After that the length was not changed.
http://ac6la.com/adhoc/VertDip2.png
Then I set up a series of test cases with the base at 0.5 ft and with the
ground characteristics varying through all the non-water choices, from
Extremely Poor to Very Good, per the EZNEC definitions of such.
http://ac6la.com/adhoc/VertDip3.png
Here's an animated gif showing the results. The value for "G" (Ground type
index) may be seen in the lower right corner.
http://ac6la.com/adhoc/VertDipB05.gif
As the ground gets better the gain at 10° gets better (pretty much). Then I
ran a similar series of test cases with the base of the dipole at 40 ft.
Again, look in the lower right corner to see the "G" value for any given
frame of the animation.
http://ac6la.com/adhoc/VertDipB40.gif
That was interesting. With the base at 40 ft the gain at 10° gets _worse_
as the ground gets better. More of the energy is going into the second
(higher) lobe. However, note that the outer ring is the same for both these
animations so you can compare magnitudes as well as pattern shapes. For any
given ground type the "DX angle" (10°) gain is always higher for the higher
dipole. For example, here's a comparison for Average ground.
http://ac6la.com/adhoc/VertDip4.png
So that seems to address the first point that VK3QI made. Gain at "DX
angles" (in this case 10°) gets better as the dipole is raised and it
doesn't matter what the ground conditions are. Now for his point about
actual measurements:
>>> Measuring a vertical at another location 5 miles away, but at the same
relative height is really measuring the ability of the vertical antenna to
couple to the ground to produce a vertically polarised ground wave.
And that seems to be in regards to page 76 from the K9YC presentation:
http://ac6la.com/adhoc/VertDip5.png
K9YC measured an increase in gain of 9.5 dB as the dipole base was raised
from 0.5 ft to 40 ft. If both he and the 3 watt transmitter at W6GJB had
been located on the wheat fields of Kansas that would have been one thing,
but they are both in the Santa Cruz mountains and I'm betting it's not
exactly 5 miles of flat ground (as NEC assumes) between them. Hence I'm not
sure how to use NEC to verify the K9YC measurements since there is no Far
Field at an elevation of 0°. So I fudged a bit.
Here's an animation with the green dot marker at 1° elevation, not exactly
the same as equal heights but close. The ground type is fixed at Very Poor
per K9YC's comments. The dipole base ranges through 0.5, 10, 20, 30, and 40
ft.
http://ac6la.com/adhoc/VertDipFF1.gif
As the base height is raised the gain at 1° increases by about 6.6 dB (from
-19.34 dBi to -12.72 dBi). That's not identical to the measured increase of
9.5 dB but it seems to verify that the gain at "ground level" (almost)
should increase as the dipole height is increased, which seems to
corroborate what K9YC measured.
Not sure what any of this proved or didn't prove, just thought I'd share it.
Here's the model file I used if anyone else would like to play with it.
(Note that you can't just click on the link to open the model. Save it to
your computer then open it from within AutoEZ, as explained in Step 3 of the
AutoEZ Quick Start guide.)
http://ac6la.com/adhoc/VertDipole.weq
This model will work just fine with the free demo version of AutoEZ which
you can download from here.
http://ac6la.com/autoez.html
Dan, AC6LA
http://ac6la.com
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